Gaseous streams containing methane, ethane and heavier hydrocarbon components occur naturally, such as in natural gas, and also as by-products of a variety of refinery processes. These gases often contain, in addition to methane and ethane, a substantial quantity of hydrocarbons of higher molecular weight, as for example, propane, butane, and pentane and their unsaturated analogs. While it is frequently desirable or necessary for industrial purposes to retain the methane and ethane content in such gases, it is not necessary to retain the higher molecular weight hydrocarbons, and it is usually economically desirable to separate some or all of such components.
In present processing, propane and the heavier hydrocarbon components of natural gas and refinery gas are separated and recovered by liquification and cryogenic distillation at temperatures below 0.degree. F. Refrigeration for separation is supplied totally or partially by expansion of the gaseous stream in a turboexpander which produces power that may be used, for example, in driving a compressor.
Prior to the advent of the cryogenic expansion process, such heavier component hydrocarbons were most frequently separated by liquification and treatment with an absorption medium. For example, natural gas streams containing propane and heavier hydrocarbon components were contacted with an absorption oil in which the liquids were absorbed and thereafter desorbed and recovered. However, with a cryogenic expansion process, it is practical to recover substantially higher percentages of the liquid hydrocarbons than with the absorption process; thus, using the cryogenic expansion process, in excess of eighty (80%) percent of the ethane content and over ninety (90%) percent of the propane and heavier components can be recovered economically. The use of absorption techniques for the separation and recovery of such hydrocarbon components therefore, is generally regarded as outdated and has fallen into disuse due to the clear economic advantages of cryogenic expansion.
In some cases, however, it is desirable to remove only part of the hydrocarbon components, as, for example, where there is no use for a particular component and, in the case of natural gas which may be unsatisfactory unless some of the heavier hydrocarbon components are retained. Specifically, ethane is often left in natural gas to increase the heating value of the natural gas and propane also may be intentionally left in the natural gas.